Are any encrypted messaging apps fail-safe?

Special counsel Robert S. Mueller III's team is reportedly reviewing the encrypted messaging apps of witnesses in the Russia investigation. The team is looking at what experts say are some of the best apps at keeping messages private. Offering people what's known as end-to-end encryption, in which only the sender and the intended recipient can read a message, the apps are designed to help people communicate more securely. They are popular among activists, journalists, security professionals and government officials.

So, how would Mueller's team be able to review the apps' contents, given the secure nature of these tools? A spokesman for the special counsel's office declined to comment on the ongoing investigation. But here's a look at how the four apps that are reportedly being reviewed work.